The present invention relates, in general, to optical force microscopes, and in particular to microscopes in which a probe is positioned and oriented by an optical trap and is scanned across an object to be inspected.
Scanning optical microscopes conventionally include a light source, a lens to focus the light on the object to be inspected, a photo detector, and a scanning mechanism to cause relative motion between the point of focus and the object. The limit of resolution of such optical microscopes is about one-half of the wavelength of the light used; that is, about 300 nm. According to the Rayleigh criterion, two neighboring points in the object are considered to be resolved in the image when the diffraction maximum of the first point coincides with the diffraction minimum of the second point. The distance between two points in the object at the limit of resolution, for incoherent illumination and a circular limiting aperture, is about 0.61 .lambda./NA, where NA is the numerical aperture of the focusing lens. In order to achieve high resolution in optical microscopes, then, a high numerical aperture objective lens is required. Oil immersion lenses of the kind commonly used in scanning optical microscopes can have a maximum numerical aperture of about 1.4.
Numerous attempts have been made in the prior art to increase the limit of resolution of scanning optical microscopes. One such attempt includes the use of an aperture with an entrance pupil positioned in the near field of the object, with the entrance pupil having a diameter smaller than the wavelength of the illuminating light. The light collected through the aperture by a photodetector is proportional to the transmissivity of the object at the location of the entrance pupil. The intensity of this collected light varies as the pupil is scanned over the surface of the object, and the resulting signal from the photodetector is recorded, with the record of the scan forming an image of the object. To achieve a resolution of about .lambda./10 with such a device requires an entrance pupil diameter of 40 nm and a separation between the entrance pupil and the object being imaged which is equal to the radius of the entrance pupil. Although such a near-field microscope has numerous advantages, its disadvantage is that the small diameter pupil has a low collection efficiency, the proximity of the pupil to the object being scanned does not permit the imaging of rough surfaces, and the aperture/object distance must be controlled by a sensitive feedback mechanism in order to avoid destructive collisions.
Another attempt to increase the resolution of microscopes is the so-called atomic force microscope which is designed to image the force between a sharp tip and an object to be inspected. Such a microscope essentially comprises a mechanical cantilevered beam carrying a sharp tip, a laser light source reflected from the cantilever, a scanner to cause relative movement between the tip and the object, and a position sensing photodetector to measure the cantilever deflection. Such an atomic force microscope provides lateral resolution approaching the atomic level (0.1 nm) on hard surfaces and a sensitivity to the force applied to the cantilever on the order of 100 pN to 1000 pN, depending on the stiffness of the cantilever and the imaging mode. The resolution of such an atomic force microscope is limited by relative vibrational motion between the tip and the object, as well as by the sharpness of the tip.
Accordingly, there is a need in the art to increase the lateral resolution available with optical microscopes by overcoming the natural limitations resulting from the dimensions of the apertures in the optical train, and to increase the force sensitivity available with force microscopes by overcoming the limitations imposed in such devices by the cantilever system.